The present invention relates to methods for attaching plates to bone. Specific examples include attaching bioresorbable plates to bone using bioresorbable fasteners.
One current methods for attaching bioresorbable plates to bone fragments is bioresorbable screws inserted with a screwdriver either manually or powered. In order to insert a screw, a threaded hole must be made into the bone. Threading or tapping is very technique sensitive and if done incorrectly the screw will not properly hold the plate to the bone. In addition, using a manual screwdriver can cause surgeon fatigue if the case requires more than a few screws to be inserted. Using a powered screwdriver speeds insertion and reduces surgeon fatigue, but can strip screws or torque off the screw head if not handled properly. The strength of standard bioresorbable screws is also in need of improvement, particularly for load bearing applications. Methods to improve the strength of resorbable screws through drawing exist, but require additional manufacturing processes and require that each screw is individually machined which is more time consuming than injection molding of standard screws. Even with these processes, the shear strength of a screw is diminished since only the minor root diameter of the threads impart the load carrying capacity. A screw that is marketed as 1.5 diameter actually only has the strength of a 1.1 diameter pin since the threads do not impart strength, but only pull out resistance.
Another method for attaching plates is using tacks or rivets. Inserting a tack is very technique sensitive. If the hole is drilled slightly oversized, a tack will not have sufficient holding power. Even if the hole is of the proper size, a tack generally does not have the same pull out resistance as screws since no threads are formed into the bone.
Eaves et al in U.S. Pat. No. 6,080,161 describe a cannulated pin that is inserted into a hole, heated and deformed in place. This method obviates the need to tap the hole and provides a means to accommodate slight variations in the diameter of the hole that is drilled. However each fastener must be individually heated adding additional time to the operative procedure. Also, the heat required to deform the fastener can add the risk of thermal necrosis to the surrounding tissue.
A relatively new method of fastener insertion is an ultrasonically inserted pin inserted using a sonotrode. This method is relatively simple, does not require tapping and requires only a minimal amount of training. The high temperatures created during insertion may induce thermal necrosis. This risk is especially pronounced at the interface of the polymer and the bone since this is where the heat is generated during insertion. Also, the molten polymer can be extruded under the plate and away from the hole during insertion since the hole that is drilled is smaller than the diameter of the fastener. Also, the fastener will often melt to the plate making removal of one individual fastener from the plate difficult.
In all of the above listed methods, the instrument must be reloaded after each fastener is inserted. This can be a time consuming process and the fastener is at times unintentionally disengaged from the instrument during this handling process. In addition, multiple lengths and diameters of fasteners must be on hand to complete each case. These fasteners are packaged in bulky packages and significant space is required to house this inventory.
A need exists for an improved fastener and method that addresses these and other concerns.